Ashley is 4 times as old as Brandon. Six years ago, Ashley was 7 times as old as Brandon. How old is Brandon now?
Solution: We can use the given information to write down two equations that describe the ages of Ashley and Brandon. Let Ashley's current age be $a$ and Brandon's current age be $b$ The information in the first sentence can be expressed in the following equation: $a = 4b$ Six years ago, Ashley was $a - 6$ years old, and Brandon was $b - 6$ years old. The information in the second sentence can be expressed in the following equation: $a - 6 = 7(b - 6)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $b$ , it might be easiest to use our first equation for $a$ and substitute it into our second equation. Our first equation is: $a = 4b$ . Substituting this into our second equation, we get: $4b$ $-$ $6 = 7(b - 6)$ which combines the information about $b$ from both of our original equations. Simplifying the right side of this equation, we get: $4 b - 6 = 7 b - 42$ Solving for $b$ , we get: $3 b = 36.$ $b = 12$.